There have been made some attempts to provide filters having a very large relative bandwidth of say about 8% or more. Ceramic filters, for example, are used for very large bandwidth filters with low losses but they are very expensive. Further, the steepness of the skirts of the transmission bands is poor in view of SAW filters.
Commonly used SAW filters work with Raleigh waves. But this type of SAW waves does show only small coupling and hence, results in wide band filters having a bandwidth of more than 20% that have big losses. This is true for all SAW filters using Raleigh waves. A greater bandwidth yields a greater loss for a given coupling. A higher coupling yields a lower loss for a given bandwidth.